1. Field of the Invention
The present invention relates to a compression glass lead-in or lead-through arrangement in a metal body, in which molten glass is introduced, such as by being fused, into an opening in the metal body, the latter of which is heated to high temperature, wherein the coefficient of expansion of the glass is lower than that of the metal, and the treating temperatures thereof lie within or lower than the range of the solidus temperature of the metal; and wherein the elastic limit of the metal may not be exceeded at any time during a subsequent cooling process.
2. Discussion of the Prior Art
Compression-type glass lead-through or lead-in arrangements of the kind considered herein are generally known in the art and are commercially available on the market in large-scale quantities; in particular for utilization for the housings of semiconductors; and even more especially, with regard to housings for integrated circuits which are adapted to be mounted or located on a substrate. The electrical terminals of the integrated circuits are to be conducted outwardly through openings formed in the housing and, in such instances, there must be present an insulation with regard to the ordinarily metallic housings. The insulative effect is provided by a glass bead which is fused into the lead-in arrangement for the housing, and which at the center thereof retains the electrical terminal conductor of the circuit at a location where it passes through the housing. Concurrently, the glass bead serves the function of hermetically sealing the interior of the housing with respect to the surroundings or outside air. Ensuring this hermetic sealing action which must be sustained during operations over a broad temperature range, is required in the production of such compression glass lead-through or lead-in arrangements.
In compression glass lead-in or lead-through arrangements of this kind, the coefficient of thermal expansion of the metal body must be substantially higher than that of the fused-in glass and that of the internal metallic conductor. After the glass has been fused into the metal body which has been heated to high temperature, the external metal portion shrinks onto the glass body during the subsequent cooling phase in consequence of its substantially higher degree or rate of contraction. As the compressive strength of various types of glass exceeds their tensile strengths at about from 10 to 20 times, that physical aspect can be utilized with regard to the process of fusing-in the compression glass, to the effect that the encountenance of tensile stresses in the glass is securely eliminated through the initial presence of a high compressive stress acting on all sides of the glass body, even in the event of the glass lead-through arrangement being subjected to severe mechanical and/or thermal loads.
The compressive stresses which are applied to the glass body due to the shrinkage of the metal portion during the cooling phase give rise in the metal part to tangentially directed tensile stresses of considerable magnitude. Any plasticity; in essence, any permanent deformation caused by the elastic limit; i.e. yield point, being exceeded must be prevented from occurrence in the metal body by means of appropriate dimensioning of tolerances in the metal body and bores, and by suitable material selection of the metal and glass. More specifically, such a phenomenon would result in that the required reserve of compressive stress would no longer be present in the event of thermal loading and which, in turn, would then result in a reduced level of thermal and mechanical resistance to shock by the compression glass lead-through arrangement, while also resulting in the latter no longer possessing the necessary vacuum-tight sealing integrity at room temperature, and without any loading thereof being involved. Accordingly, it is necessary for the level of any tensile strength and the elastic limit of the metal employed to be sufficiently high.
Heretofore, iron-nickel alloys and steels have essentially been employed for such compression glass lead-in or lead-through arrangements. Titanium alloys have also been used at this time for special applications; namely for the housings of cardiac pacemakers; nevertheless, such titanium alloys, by virtue of the particular physiological requirements involved, must possess properties which are evidently different from those which are required with regard to ordinary metal bodies of this type.
The mentioned alloys of iron-nickel and steel have an elastic limit of &gt;200 N/mm.sup.2 and a coefficient of thermal expansion .alpha. of about 80-180.times.10.sup.-7 /.degree.C. The latter is markedly different from the coefficient of thermal expansion of the appropriately used kinds of glass, which is about 45-100.times.10.sup.-7 /.degree.C. The foregoing materials possess the advantage in that they regain their high elastic limit during the cooling phase after the fusing-in procedure, and by virtue of the different coefficients of thermal expansion relative to glass, can still exert a highly excessive compressive stress on the glass body, even during the cooling phase and also at room temperature.
However, materials of the above-indicated kind no longer fulfill the ever increasing requirements of industry which are placed thereon with respect to contemplated future housings for semiconductors and which, in particular, are characterized by a higher level of thermal conductivity, ease of machinability, and a high degree of resistance to encountered fluctuations in temperature, and can be produced at low cost.
Copper and aluminum alloys which are ordinarily commercially available and which to some extent possess such desirable properties; are nevertheless subject to such a low elastic limit that they already encounter plastic deformations during the cooling phase, and as a consequence, the compression glass lead-through arrangement can no longer be produced to be able to facilitate a hermetically-sealed condition.
In addition to the above-described kind of assembly involving metal-glass fusion, in effect, producing the compression glass lead-in or lead-through arrangement, there is also presently known the so-called "matched" lead-through arrangement, which is not the subject matter of the present invention but which is referred to herein for purposes of presenting a comparison with regard to the advantages of the invention. That other arrangement is characterized in that all of the cooperating components which are to be fused; in effect, the outer metal portion, glass body, internal metallic conductor, are `matched`; in essence, their coefficients of thermal expansion remain approximately the same over a wide temperature range. The result of this is that, with such kind of utilization, the occurrence of stresses need not be considered as a result of different degrees of contraction during the cooling phase, and accordingly the elastic limit of the outer metal portion does not represent any crucial criterion with regard to the design of such a housing.
Currently, iron-nickel-cobalt alloys which are commercially available under the tradename "Kovar" are used for such `matched` lead-through arrangements; in the so-called `low-level matched` design configuration. The alloys have a coefficient of thermal expansion of approximately 45-55.times.10.sup.-7 /.degree.C. The coefficient of expansion of the various types of glass used for that purpose, and that of the metallic inner conductor, is of the same order of magnitude.
The use of Kovar provides the advantage that the thermal load-carrying capacity, such as the temperature and shock resistance, of a housing produced from that material is considerably higher than that found for the normal compression-type glass lead-through arrangements. Therefore, recourse is preferably had to this design which, in the interim, has proven itself over a lengthy period of time, in particular where there is a need for the presence of a high degree of reliability for the component in connection with critical temperature influences.
An alternative form of the `high-level matched` lead-through arrangement is also conceivable and has been produced to some minor extent in commerce. In that arrangement, the cooperating fusion components once again possess approximately equal coefficients of thermal expansion, however, the absolute value u is not within the range of 80-100.times.10.sup.-7 /.degree.C. Lead-through arrangements with still higher coefficients of thermal expansion can also be conceived, but are not employed in actual practice because, on the one hand, the kinds of glass required for that purpose afford only a low level of chemical resistance and inadequate insulating properties and, as a result, are not suitable for their reliable employment, while on the other hand, because of the fact that the resistance to fluctuations in temperature drops off severely at rising coefficients of expansion of the matched material pairings.
Consequently, the advantage of obtaining a good resistance to fluctuations in temperature is only encountered in the case of `low-level matched` lead-in or lead-through arrangements. Housings of "Kovar" are currently in use, although they are substantially more expensive than compression glass lead-in arrangements, and even though they are subject to the not inconsiderable disadvantage of possessing a poor thermal conductivity, and with the material thereof being more difficult to machine.
It is therefore desirable to be able to produce a compression glass lead-in arrangement from a material which is easier to handle and process than Kovar and which, on the one hand, distinguishes itself through improved physical properties while, nonetheless, fulfilling all critical requirements placed thereon and, on the other hand, being an equal match to Kovar with regard to resistance to fluctuations in temperature.